Submesh residuum output for TH2M and TRM processes

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented? -> see #3356

TODOs

• Extend non-stationary TH2M test with submesh residuum output (e.g., Ogata-Banks) -> Follow-up MR, see #3356
• Cleanup code
• Discuss possibly necessary changes, e.g., in prj file configuration
• unify names: residual -> residuum
• create ctests
• check that this MR does not break PETSc, at least if no submesh assembly is done.

For the PETSc test see:

AddTest(
    NAME ThermoRichardsMechanics_LiakopoulosMixedElementsPETSc
    PATH ThermoRichardsMechanics/LiakopoulosPETSc
    EXECUTABLE ogs
    EXECUTABLE_ARGS liakopoulos_mixElem_mumps.prj
    WRAPPER mpirun
    WRAPPER_ARGS -np 2
    TESTER vtkdiff
    # Run on envinf only because PETSc MUMPS solver is used
    REQUIREMENTS OGS_USE_PETSC AND "${HOSTNAME}" MATCHES "envinf.*"
    RUNTIME 2
    DIFF_DATA
    GLOB liakopoulosBulk_mixElem_t_*.vtu sigma sigma 1e-9 1e-12
    GLOB liakopoulosBulk_mixElem_t_*.vtu displacement displacement 1e-10 1e-12
    GLOB liakopoulosBulk_mixElem_t_*.vtu saturation saturation 1e-10 1e-12
)

The PETSc test runs, indeed:

321/337 Test #300: ogs-SteadyStateDiffusion_cube_1x1x1_Neumann_1e5-mpirun-vtkdiff .............................................   Passed    0.16 sec
322/337 Test #222: ogs-ThermoRichardsMechanics_LiakopoulosMixedElementsPETSc-mpirun ...........................................   Passed   67.01 sec
        Start 223: ogs-ThermoRichardsMechanics_LiakopoulosMixedElementsPETSc-mpirun-vtkdiff
323/337 Test  #57: snakemake_reorder_mesh .....................................................................................   Passed   55.88 sec
324/337 Test #223: ogs-ThermoRichardsMechanics_LiakopoulosMixedElementsPETSc-mpirun-vtkdiff ...................................   Passed    1.56 sec